1. Field of the Invention
This invention relates to neutron generating systems and more particularly pertains to a new and improved neutron generator especially adapted to traverse the narrow confines of a well or borehole, although useful in a variety of other applications. Since a neutron generator embodying the invention is ideally suited to the needs of well logging services, it will be described in that connection.
2. The Related Art
The use of a generator of high energy neutrons has been known for a long time for neutron-gamma ray or neutron-neutron logging. A neutron generator has advantages compared with chemical neutron sources, in particular it features a negligible amount of radiation other than the desired neutrons; a high yield of neutrons; a controllable yield of neutrons in bursts or continuously; neutrons at higher energies than formerly possible; mono-energetic neutrons; and control of the generator so as to permit its deactivation prior to withdrawal from or insertion in a well. The first five of these attributes are important in obtaining more informative logs, while the last is valuable in minimizing health hazards to operating personnel.
Neutron generators used in oil well logging tools usually require controlled low pressure atmospheres and high intensity magnetic fields. Accordingly, for illustrative purposes, the invention is described in more complete detail in connection with a neutron generator suitable for use in a well logging tool.
Neutron generators usually have three major features:
(i) a gas source to supply the reacting substances, such as deuterium (H.sup.2) and tritium (H.sup.3); PA1 (ii) an ion source comprising usually at least one cathode and an anode; electrons are emitted from the cathode surface when an electrical impulse is applied to the anode; impact of the primary electrons on the gas molecules result in subsequent secondary electrons being stripped from the gas molecules, thus generating positively charged ions; and PA1 (iii) an accelerating gap which impels the ions to a target with such energy that the bombarding ions collide with deuterium or tritium target nuclei in neutron (n) generating reactions: EQU H.sup.2 +H.sup.2 .fwdarw.He.sup.3 +n+3.26 MeV EQU H.sup.2 +H.sup.3 .fwdarw.He.sup.4 +n+17.6 MeV EQU H.sup.3 +H.sup.3 .fwdarw.He.sup.4 +2n+13 MeV PA1 where He.sup.3 and He.sup.4 are helium isotopes, and the energy is expressed in millions of electron volts (MeV). PA1 1) a sonde incorporating at least one radiation detector; and PA1 2) a neutron generator comprising: PA1 an ion source comprising an anode and a dispenser or volume type cathode disposed in an ionizable gas environment including at least one hydrogen isotope; PA1 means for heating the cathode so that the latter emits electrons which, when colliding with the gas atoms, generate ions; PA1 a target; PA1 an electrical gap to accelerate ions from the ion source towards the target upon impingement of the ions; and PA1 control means for applying voltages to the anode, cathode and electrical gap.
Ordinarily, negative electrons and positively charged ions are produced through electron and uncharged gas molecule collisions within the ion source. Electrodes of different potential contribute to ion production by accelerating electrons to energy higher than the ionization threshold. Collisions of those energetic electrons with gas molecules produce additional ions and electrons. At the same time, some electrons and ions are lost to the anode and cathode. In this manner, the positive and negative charges inside the ion source approach an equilibrium. Collision efficiency can be increased by lengthening the distance that the electrons travel within the ion source before they are neutralized by striking a positive electrode. One known path lengthening technique establishes a magnetic field which is perpendicular to the aforementioned electric field. The combined magnetic and electrical fields cause the electrons to describe a helical path within the ion source which substantially increases the distance traveled by the electrons within the ion source and thus enhances the collision efficiency of the device.
This type of ion source, called "Penning ion source", is part of a family of "cold cathode ion sources" and has been known as early as 1937; see for example the article by F. M. Penning and J. H. A. Moubis in Physica 4 (1937) 1190. Examples of neutron generators including such "cold cathode ion source" used in logging tools are described e.g. in U.S. Pat. No. 3,546,512 or 3,756,682 both assigned to Schlumberger Technology Corporation.
However, neutron generators using Penning ion sources used in logging tools suffer from several drawbacks.
First, the anode being at a high potential, in the range of 1 to 3 kV, the cathode suffers erosion due to energetic ion bombardment. Material sputtered from the cathode may coat the insulator surfaces provided for electrical insulation either of the anode or of the target. This may cause instability which is prejudicial to the operation of the ion source. Also, this instability occurring in a space where high voltages are involved can be detrimental to safety.
Second, most logging nuclear measurements are carried out by emitting pulses of neutrons which irradiate the earth formations, and by detecting the radiation (neutrons or gamma rays) resulting from the interaction of earth formation atoms and the emitted neutrons. Thus, it is critical to have a good knowledge of the characteristics of the neutron pulse, such as the neutron output (number of neutrons emitted) and the pulse timing. Such knowledge means having control over these characteristics. It is highly desirable to generate neutron pulses having a substantially square shape, in particular a short rise time (to reach the plateau value) and a short fall time (once the voltages are turned off). However, in a Penning source, such tasks are difficult because the charge populations in the source, particularly the electron population, do not reach equilibrium instantaneously; see F. Chen, J. Appl. Phys. 56 (11) 3191, 1984. The rate at which the charge populations approach the equilibrium depends strongly on the gas pressure in the source. This effect manifests itself in the slow rise time of the neutron pulse, and a delay, typically a few microseconds (although sometimes variable with operation conditions), between the time the voltage appears at the anode and the start of the neutron pulse. Since the cathode and anode surface conditions are not identical between different neutron tubes, different pressures are often required to achieve the same neutron output. This makes the timing control of the source all the more difficult that it is essentially a function of the particular neutron generator, and may vary over the operating lifetime of the neutron generator.
Third, the high voltage required for a Penning ion source (1-3 kV) is generally produced via a pulse transformer. The transformers are designed for a certain pulse width. Thus, changing pulse length results in altering the performance, most noticeably, the neutron pulse shape. There have been some attempts to improve the neutron pulse shape generated from a cold cathode ion source. In particular, the article "Neutron Generators for Wireline Application," from R. Ethridge et al., 1990 IEEE Nuclear Science Symposium Conference Record, Arlington, Va., Oct. 22-23, 1990, Vol. 2 of 2, describes a cold cathode source wherein the pulse transformer is provided with a "clamping" circuit designed to decrease the fall time of the pulse. However, such clamping circuits: (i) do not seem to improve the rise time of the neutron pulse; (ii) require additional power; (iii) and increase the overall size of the control circuit.
Fourth, the known cold cathode sources can usually operate in any one of several discharge modes according to the relative ion and electron populations and different plasma sheath structures. The anode voltage, the magnetic field and the gas pressure determine the operating point at which the production and loss of electrons and ions are at balance. In addition, under certain conditions, the operating point is unstable near certain mode boundaries. The transition from one mode to the other can lead to a substantial change in the ion beam density and electron extraction efficiency, and, with control circuits currently used that regulate the beam current by lowering the gas pressure, to a reduction in gas pressure that can result in oscillations about the mode boundary. The resulting neutron output variations are detrimental to the overall quality of the measurements.
Fifth, the means for generating the magnetic field, intended to lengthen the electrons path, are relatively cumbersome and increase the overall dimensions and weight of the neutron generator. This is of concern in a logging tool where room is limited.
An alternative to the cold cathode ion sources are "hot cathode" ion sources, proposed as early as 1939, associated to a spectrograph, as depicted e.g. in the article "Focused Beam Source of Hydrogen and Helium Ions" by G. W. Scott Jr., in Physical Review, May 15, 1939, volume 55. Further developments in the same technical area provided some modifications to the basic hot cathode ion source; see e.g. the article "An Electrostatically Focused Ion Source and its Use in a Sealed-Off D.C. Neutron Source" by J. D. L. H. Wood and A. G. Crocker, Nuclear Instruments and Methods, 21 (1963) pages 47-48; or the article "Electron Bombardment Ion Source for Low Energy Beams" by S. Dworetsky et al., in The Review of Scientific Instruments, November 1968, volume 39, No. 11. A "hot cathode" typically comprises a material susceptible, when heated, to emit electrons. The cathode is disposed above, or concentrically to, the anode. An extracting electrode (also called focusing electrode) is placed at the front of the anode to extract ions, generated from collisions between electrons and gas molecules, and focus such ions so as to form an ion beam.
Hot cathode ion sources by themselves bring some improvements with respect to cold cathode ion sources. Hot cathode sources for instance: (i) do not always require a magnetic field, and this allows a substantial reduction in weight and dimensions; (ii) are able to generate an optimum electron flux in a relatively short period of time after the voltage pulse is applied to the anode; (iii) as being used in sealed neutron generator, do not show troublesome mode transitions in the range of gas pressure where these devices normally operate; and (iv) do not require a high anode or cathode voltage when used in neutron generators including a discharging gas made of deuterium and tritium; this reduced voltage entails a reduction in electrode erosion.
However, hot cathode ion sources present drawbacks of their own compared to cold cathode ion sources, such as: (i) additional power; (ii) a relatively reduced lifetime at least for most of hot cathode filament materials, and (iii) the need for a specific structure to support the hot cathode and anode, especially in view of the severe shock and vibration conditions encountered during logging operations.
Moreover, according to applicant's knowledge, the known hot cathode ion sources were implemented in laboratories and designed mainly for experimental purpose, which applications are not subjected to the severe environmental constraints typical of the logging techniques. In other words, performances of these known hot cathode ion source could be considered sufficient for laboratory measurements but would not be acceptable for logging applications, even assuming they could be directly implemented in a logging tool. Among others, one could mention, as constraints specific to logging applications: weight and dimensions, safety, neutron pulse shape, neutron output, power requirements, and operating lifetime.
Accordingly, although the neutron generators used so far in the logging techniques have been working relatively satisfactory, there still is a need for an improvement to the neutron output and especially to the neutron pulse shape.